1. Field of the Invention
The present invention relates to a mobile communication system using an Orthogonal Frequency Division Multiple Access (OFDMA) scheme, which will be referred to as an OFDMA mobile communication system, and more particularly to a method for transmitting or receiving data by means of differential combinations of signal transmitting and receiving schemes according to data characteristics.
2. Description of the Related Art
In a 4th generation (4G) mobile communication system, which is the next generation mobile communication system, research has been actively pursued to provide users with services having various qualities of service (QoS) at a transmission speed of about 100 Mbps. Apart from previous mobile communication systems providing only wireless mobile communication services, the 4G mobile communication system is aimed at providing integrated wire/wireless communication services by efficiently combining a wire communication network with a wireless communication network. As a result, the wireless communication network requires technology capable of transmitting mass storage data at the same or similar rate and capacity of a wire communication network.
In the 4G mobile communication system, an Orthogonal Frequency Division Multiple (OFDM) scheme has been actively researched for transmitting data through wire/wireless channels at high speeds. The OFDM scheme, which transmits data using multi-carriers, is a special type of a Multi-Carrier Modulation (MCM) scheme in which a serial input symbol sequence is converted into parallel symbol sequences and the parallel symbol sequences are modulated with a plurality of mutually orthogonal sub-carriers (or sub-carrier channels) before being transmitted. The OFDM scheme is similar to a Frequency Division Multiplexing (FDM) scheme, but it can achieve optimal transmission efficiency in high speed data transmission by transmitting a plurality of sub-carriers while maintaining orthogonality therebetween. Further, the OFDM scheme is quite efficient in its use of frequencies and is tolerant to multi-path fading, so that it can achieve optimal transmission efficiency in high speed data transmission.
Furthermore, the OFDM scheme has advantages as follows: it is efficient in its use of frequencies by using an overlapping frequency spectrum; it is tolerant to frequency selective fading; it can reduce Inter-Symbol Interference (ISI) by means of a guard interval; it enables a hardware structure of an equalizer to be simply designed; and it is tolerant to impulse noise. Therefore, the OFDM scheme has been generally employed in a mobile communication system.
According to the OFDM scheme, spectrums between sub-carriers (i.e., sub-carrier channels) overlap while maintaining mutual orthogonality, so that spectrum efficiency is good. Further, in the OFDM scheme, modulation is accomplished by an Inverse Fast Fourier Transform (IFFT) and demodulation is accomplished by a Fast Fourier Transform (FFT). A multiple access scheme based on the OFDM scheme as described above includes an OFDMA scheme which allocates some of entire sub-carriers to a specific terminal for use. The OFDMA scheme does not require a spreading sequence for band-spreading and may dynamically change a set of sub-carriers allocated to a specific terminal according to a fading characteristic of a wireless transmission line.
As a result, the 4G mobile communication system as described above has been developed in consideration of a software aspect for developing various contents and a hardware aspect for developing a wireless access scheme having high spectrum efficiency to provide best QoS.
Hereinafter, the hardware aspect considered in the 4G mobile communication system will be described.
Generally, factors obstructing a high speed service of high quality in wireless communication are caused by channel environments. In the wireless communication, the channel environments frequently change due to power variation of received signals caused by fading in addition to Additive White Gaussian Noise (AWGN), shadowing, a Doppler effect by movement and frequent velocity variation of a terminal, or interference by other terminals and multi-path signals. Accordingly, to provide a high wireless data packet service, another developed technology capable of adaptively coping with the channel change has been required in addition to schemes provided by the existing 2G or 3G mobile communication system. For example, a scheme (e.g., an Adaptive Modulation and Coding (AMC) scheme and a Hybrid Automatic Retransmission Request (HARQ) scheme) employed in existing mobile communication systems adaptively copes with the channel change, thereby improving entire performance of the system greatly.
Even though various schemes including the AMC scheme and the HARQ scheme as described above are employed, a fundamental problem (i.e., shortage of radio resources) in wireless communication is not solved. Accordingly, it is desirable to continuously research and develop a multiple access scheme having high spectrum efficiency, which enables subscriber capacity to be maximized and high speed transmission indispensable for a multimedia service to be accomplished. Further, it is desirable to provide a new multiple access scheme which has good spectrum efficiency and can consider data characteristics to provide a high speed packet data service of high quality.